1. Field of the Invention
The present invention relates to a shake detection system using a high-pass filter arithmetic means, and more particularly, to a camera shake detection apparatus for detecting a shake in a photographing apparatus or the like, and a photographing apparatus such as a camera having a camera shake prevention function, which uses the shake detection apparatus to prevent an image degradation caused by a camera shake.
2. Description of the Related Art
An acceleration sensor, an angular velocity sensor, or a rate gyro is conventionally used as a camera shake detection apparatus for a photographing apparatus such as a camera. To make a compact equipment, these shake detection apparatuses are also downsized.
However, when a shake detection unit is made compact, an output signal contains drift and offset components because of a change in ambient temperature or an increase in temperature of the element itself in use, resulting in a degradation in output precision. As one of the smallest elements, a shake detection unit constituted by a piezoelectric element is proposed. In this case, however, not only the element is deformed by a change in temperature, but also a large drift component is produced by a change in capacitance.
FIG. 10 is a graph showing a change in signal in starting a shake gyro type angular velocity sensor serving as a shake detection sensor. Referring to FIG. 10, time is plotted along the abscissa, and an output from the shake gyro type angular velocity sensor is plotted along the ordinate. According to FIG. 10, the sensor in a stationary state exhibits very unstable signal characteristics at the start time during several tens msec (50 msec is typically exemplified in FIG. 10) after the power-on operation. During this period of time, the sensor output largely changes to an output level near that in the absence of a shake (so-called null voltage).
Thereafter, an unstable initial period of several hundreds msec (300 msec is typically exemplified in FIG. 10) starts. During this period of time, the sensor output gradually changes from a level near the predetermined null voltage to a null voltage output. Thereafter, a stable period starts, and the sensor output does not largely change during this period of time.
Referring to FIG. 10, a broken line indicates sensor output characteristics in a camera shake superposed state.
FIG. 11 is a graph showing a change in signal when a longer time has elapsed. Referring to FIG. 11, although the sensor is in the stable period, the null voltage changes. This is because the sensor output generates a drift due to a change in ambient temperature and the like. The change in null voltage due to the drift of the shake sensor, which is mainly considered to be caused by a change in temperature, causes a shake signal error although the change is very moderate at a period of several minutes.
To remove this drift component, U.S. Pat. No. 4,623,930 uses a high-pass filter to remove the low-frequency component of the angular velocity sensor, which is generated by the shake gyro. When a high-pass filter is used, a so-called cutoff frequency for determining the upper limit of the frequency to be removed by the filter is inversely proportional to the time constant of the filter. In starting the sensor or the filter, a time corresponding to the time constant, i.e., a time of 2.pi./f (f is the upper limit frequency to be cut off) is necessary until the signal is stabilized. Particularly, at the start time, a longer time is required to remove the error because of noise or the unstable sensor.
FIG. 12 is a block diagram of shake detection apparatus constituted by the conventional typical shake gyro type angular velocity sensor. Referring to FIG. 12, a high-pass filter (HPF) 2 removes a drift component in an output from a shake gyro type angular velocity sensor 1. This signal is amplified to a desired signal level by an amplifier 3. The amplified signal is adjusted by a voltage regulator 4 such that the offset output from the amplifier 3 is canceled, or a predetermined voltage is obtained as an output in the absence of shake. The adjusted signal is output and used as a shake signal.
In measuring a shake in the photographing apparatus to prevent a camera shake in the photographing apparatus, the camera shake has a frequency of about 15 Hz even on the high-frequency side. To the contrary, the amplitude of the shake on the low-frequency side becomes large. Therefore, to remove the sensor drift without attenuating the camera shake signal, a high-pass filter (HPF) with a large time constant is required. Particularly, in a still camera, a moderate shake in frequency is not visually corrected by the photographer unlike a video camera. Since all shakes are photographed onto one frame of the film, a shake at a lower frequency must be detected, and an HPF with a time constant of several tens sec is required.
FIG. 13 is a graph showing transition of the shake signal after starting the arrangement in FIG. 12. The sensor output value before the start is largely different from that after the start, and an input to the HPF largely changes during an initial period. For this reason, a long time corresponding to the time constant is necessary until the HPF output is converged.
In, e.g, Jpn. Pat. Appln. KOKAI Publication No. 63-50729, an integrator having an HPF capable of setting a plurality of time constants is used to integrate outputs from the acceleration sensor, and the time constants are selectively used, thereby decreasing the time required until the filter is stabilized at the start time.
Jpn. Pat. Appln. KOKAI Publication No. 63-275917 discloses an HPF for switching an analog high-pass filter consisting of a CR element from a small time constant state to a large time constant state in starting the sensor by using an impedance element and a switching element.
In the above methods, however, the sensor output value before the start is largely different from that after the start, as shown in FIG. 14. Since an input to the HPF largely changes during an initial period, a long time is required until the HPF output is converged although the effect for improving the convergence is provided.
In U.S. Pat. No. 5,245,378, a technique is described in which an alarm is issued or a release operation is inhibited before the start until the sensor output is stabilized.
However, to control the time constant and enable early rising of the HPF, the sensor must be in a stable state. More specifically, assume that, when a shake is applied to the sensor, e.g., when the photographer holds the camera by hands, the sensor is powered on, or the HPF starts to operate. In this case, if the time constant of the HPF is small, and the sensor output on the high-frequency side can be easily cut off, an offset component is contained in the HPF output upon shifting to a stationary state.
Not only at the start time, but also when a large signal is continuously input, the filter tends to have an offset component. When the photographer slowly moves the camera to track an object in a finder or to change a view angle, i.e., when the shake signal is small, the HPF removes even this moderate change in signal as far as the signal is continuously input for a long time. In this case as well, when the photographer stops moving the camera for a photographing operation, the change in signal at the stop time is faster and larger than the change so far. Therefore, the HPF output inversely produces an offset component.
For this reason, simple control of the time constant at the start time cannot provide its effect when the HPF output produces an offset component.
In addition, even when an alarm is generated, or a release operation is inhibited at the start time until the sensor is stabilized, no effect is provided in an actual photographing operation by the photographer.